High performance superalloys, such as nickel-or cobalt-based superalloys, are being increasingly employed in various types of gas turbines used, for example, in the propulsion and power generation industries. Blades of high performance and high fuel efficiency turbine engines generally having a hollow core made of nickel- or cobalt-based superalloy are subjected to corrosive exhaust gases at extremely high temperatures of up to 1150.degree. C. during the operation of such turbines. As a result, these blades are prone to oxidation damage. Various means have been tried to prevent such oxidation damage.
One means typically employed is to provide these blades with an environmentally resistant surface region of an aluminum-rich alloy, such as aluminide, whose surface oxidizes to form an aluminum oxide (alumina) scale at elevated temperatures. Such a scale provides a tough, adherent layer that is highly resistant to oxidation and corrosion attack. One example of such a protective diffusion layer is disclosed in U.S. Pat. No. 3,677,789 where nickel- or cobalt-based superalloy is provided with a coat of metallic aluminum alloyed with at least one metal of the platinum group.
In a typical aluminide coating process suitable for coating nickel- and cobalt-based superalloy surfaces, there are conflicting needs in terms of the desired aluminum content in the coating resulting therefrom. An aluminide region having more than 50 atomic percent aluminum is exceedingly desirable for producing a highly oxidation-resistant surface. However, when such a high concentration of aluminum is present, aluminum diffuses into the nickel-or cobalt-based superalloy substrate to form a nickel or cobalt aluminide region. Such a diffusion process is termed an inward diffusion process.
The presence of other superalloy constituents, especially refractory elements such as tungsten, tantalum and molybdenum, in the aluminide region adversely affects the degree of oxidation protection imparted by said region to the underlying superalloy substrate. The aforementioned problem can be somewhat alleviated by having an aluminide region with less than 50 atomic percent aluminum. When such a low concentration of aluminum is present in the aluminide region, nickel diffuses outward into said region, significantly limiting the presence of the aforementioned refractory elements therein. Such a diffusion process is termed an outward diffusion process. However, such a low concentration of aluminum in an oxidation-resistant aluminide region severely reduces its effectiveness since the amount of aluminum available for producing an oxidation-resistant alumina scale is small.
These diffusion phenomena create a situation where the various regions of the metals are not pure superalloy or pure aluminide, but rather mixtures of these and other materials in varying degrees. It is for that reason that they are referred to herein as "regions" rather than as layers of one metal or metal compound or another. It should be understood, however, that said regions are produced by techniques typical of those employed to produce layers of various metals in other contexts.
Thus, there exists a need for producing an effective oxidation-resistant aluminide surface region that will address the aforementioned concerns simultaneously by controlling the thermodynamic activity of aluminum during the deposition process.